It has heretofore been tried to produce mufflers using synthetic resins for overcoming the drawbacks of metallic mufflers such as heavy weight and corrosion. However, none of such synthetic resin mufflers are satisfactory yet. For example, a synthetic resin muffler reinforced by carbon fibers or glass fibers or a muffler having an outer shell formed of a synthetic resin is disclosed in Japanese Patent Laid-Open No. 75521/1980. But normal heat-resistant temperature of the synthetic resins proposed heretofore are lower than 150.degree. C. Besides, since these resins are thermosetting resins, it is impossible to adopt an efficient munufacturing method such as injection molding. Further, it is required as an essential condition to use a heat insulating material between such outer shell and an end member. In the event of heat transfer to the outer shell, the outer shell will be thermally deteriorated and no longer fulfil its function as a muffler.